The word “printer” as used herein encompasses any apparatus, such as a digital copier, book marking machine, facsimile machine, multi-function machine, etc., that produces an image with a colorant on recording media for any purpose. Printers that form an image on an image receiving member and then transfer the image to recording media are referenced in this document as indirect printers. Indirect printers typically use intermediate transfer, transfix, or transfuse members to facilitate the transfer of the image from the image receiving member to the recording media. In general, such printing systems typically include a colorant applicator, such as a printhead, that forms an image with colorant on the image receiving member. Recording medium is fed into a nip formed between the surface of the image receiving member and a transfix roller to enable the image to be transferred and fixed to the print medium so the image receiving member can be used for formation of another image.
A schematic diagram for a typical indirect printer that includes a printhead that ejects phase change ink on the image receiving member to form an image on the member is illustrated in FIG. 7. The solid ink imaging device, hereafter simply referred to as a printer 110, has an ink loader 112 that receives and stages solid ink sticks. The ink sticks progress through a feed channel of the loader 112 until they reach an ink melt unit 114. The ink melt unit 114 heats the portion of an ink stick impinging on the ink melt unit 114 to a temperature at which the ink stick melts. The liquefied ink is supplied to one or more print heads 116 by gravity, pump action, or both. Printer controller 122 uses the image data to be reproduced to control the print heads 116 and eject ink onto a rotating print drum or image receiving member 140 as image pixels for a printed image. Recording media 120, such as paper or other recording substrates, are fed from a sheet feeder 118 to a position where the image on the drum 140 can be transferred to the media. To facilitate the image transfer process, the media 120 are fed into a nip between the transfer, sometimes called transfix roller 150, and the rotating print drum 140. In the nip, the transfix roller 150 presses the media 120 against the print drum 140. An assembly 124 of lever arms, camshafts, cams, and gears urged into motion by an electrical motor responds to signals from the controller 122 to move the transfix roller into and out of engagement with the print drum 140. Indirect or offset printing refers to a process, such as the one just described, of generating an ink or toner image on an intermediate member and then transferring the image onto some recording media or another member.
To optimize image resolution in an indirect printer, the conditions within the nip are carefully controlled. The transferred ink drops should spread out to cover a specific area to preserve image resolution. Too little spreading leaves gaps between the ink drops while too much spreading results in intermingling of the ink drops. Additionally, the nip conditions are controlled to maximize the transfer of ink drops from the image member to the print medium without compromising the spread of the ink drops on the print medium. Moreover, the ink drops should be pressed into the paper with sufficient pressure to prevent their inadvertent removal by abrasion thereby optimizing printed image durability. Thus, the temperature and pressure conditions are carefully controlled and must be consistent over the entire area of the nip.
When an indirect printer, such as the one shown in FIG. 7, is powered on, the image receiving member needs to be heated to a predetermined temperature that enables the melted phase change ink to remain on the surface of the image receiving member, yet be malleable enough for transfer and fixing to the recording media when the ink image enters the nip. An image receiving member with a larger thermal mass requires more thermal energy and more time to reach the predetermined temperature than an image receiving member that has a smaller thermal mass. In an effort to reduce the time required for an image receiving member to reach the predetermined temperature, the wall of an image receiving member has been reduced in thickness. While this reduction in wall thickness does decrease the time required for the image receiving member to reach the predetermined temperature, it also affects the pressure conditions in the nip formed with the transfix roller.
Without a change in the transfix roller, the pressure in the nip becomes less uniform and weaker in the center of the nip between the ends of the transfix roller and the image receiving member. As shown in FIG. 6, a nip formed with an image receiving member having a 9 mm thick wall has one pressure profile from one end to the other end of the nip across the width of the transfix roller and image receiving member, while a nip formed with an image receiving member having a 4.5 mm thick wall has another profile. The pressure profile for the thin wall member has a pressure at each end of the profile that is greater than the pressure at each end of the profile for the thick wall member. Additionally, the pressure in the center of the thin wall member profile is substantially below the pressure in the center of the thick wall member profile. These pressure differences are likely to cause wrinkles in the recording media in the nip and the print quality suffers from the lack of consistency in the pressure across the width of the nip. Enabling the nip conditions to help ensure the print quality is adequate and the media is not distorted with thinner wall image receiving members is a desirable goal.